If you are currently jailbroken and want to update, you will have to backup, do a complete restore with iTunes to iOS 6.1.2, activate and recover your backup then jailbreak with evasi0n. Of course, this sounds like a lot of ‘work’ so only do so if you really want to get on iOS 6.1.2.

Update: evasi0n 1.4 is coming later this afternoon to support iOS 6.1.2:

We will release evasi0n 1.4 later today that marks iOS 6.1.2 as supported.